The present invention relates to interference suppression systems and more particularly to side-lobe canceller systems which are compatible with MTI.
Generally, interference suppressing systems of the prior art are designed to reduce the presence of undesired signals in a signal receiving system. As is known, in particular systems, such as a radar system, the characteristics of the receiving antennas are such that undesired signals which are received in the side-lobe interfere with the isolation of the target signal received in the main lobe. Accordingly, to isolate the main lobe signals, side-lobe cancellers have been used to cancel the side-lobe interference of the main radar antenna as exampled by U.S. Pat. No. 3,202,990 to Paul W. Howells.
It has been found that while conventional side-lobe canceller systems are highly successful in most instances, problems arise when the system is used to cancel interference in a high clutter environment where normally Moving Target Indicators (MTI) can be used to cancel the clutter. An MTI relies on the clutter return being the same on a pulse to basis in order to allow distinction from a target return which varies due to target movement between radar pulses. When clutter is present simultaneously with jamming interference, however, it has been found that conventional side-lobe cancellers modify the clutter returns on pulse to pulse basis because of cross modulation between clutter and interference signals. As the clutter returns are not the same, an MTI cannot be used thereby preventing clutter cancellation and prohibiting the use of a side-lobe canceller with the MTI.
One proposal for avoiding clutter modification in a side-lobe canceller involves the sampling of the interference signal in the correlator loop for a short time immediately preceeding radar pulse transmissions, where clutter is weak or non-existent, and using the derived weighting signal to cancel throughout the next pulse repetition period. Such a technique, while reducing clutter modification, also introduces time dependent unbalance in the cancelling signals due to antenna scan between samples.
In U.S. Application Ser. No. 05/499,962 entitled "MTI COMPATIBLE COHERENT SIDELOBE CANCELLER" to Bernard L. Lewis, filed in the United States on Aug. 23, 1974 now U.S. Pat. No. 5,296,865 and assigned to the same assignee as the present application, a side-lobe canceller system is disclosed which delays both main and auxiliary channel signals until cancelling weights at the beginning and end of a pulse repetition period can be derived. Interpolation between the two weights as a function of time is then performed over the pulse repetition period to provide an instantaneous weighting function for translating the delayed auxiliary channel interference signal for subtraction from the main channel interference signal. While such a system provides improved cancellation, the technique suffers from the disadvantage of requiring long delays of wide-band signals to be accurately matched in two separate channels.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an improved predicting coherent side-lobe canceller system for producing reliable interference cancellation in a high clutter environment.